1. Field of the Invention
The invention relates to flash analog-to-digital converters (ADCs), and more particularly to offset calibration of flash analog-to-digital converters.
2. Description of the Related Art
Currently, analog-to-digital converters (ADCs) are widely used in a variety of applications, such as medical systems, audio systems, test and measurement equipment, communication systems, and image and video systems. The most common ADC construction comprises flash ADCs, pipeline ADCs and successive approximation register (SAR) ADCs, wherein the flash ADCs and the pipeline ADCs are faster than the SAR ADCs.
High-speed low-resolution flash ADCs are widely used in current circuit systems, such as these for disk drive read channels, DVD playback, and communication receiver systems. In general, an ADC converts a continuous quantity (e.g., voltage) into a digital representation (e.g., binary code that represents the voltage). Theoretically, an ideal ADC has a one-to-one mapping, also known as its transfer function. For example, an ideal ADC converts a unique voltage value into a unique digital code. Non-idealities can arise from environmental changes, such as changes in power supply voltage or operating temperature, or operational changes, such as changes in input signal voltages, converter resolutions or offset error.
An ADC may comprise multiple comparators for conversion of the analog data into digital data. In order to accurately convert the analog data, it is desirable that the comparators exhibit very little electrical variation from ideal operation even in the presence of “offsets”. Many sources exist for offsets including mismatch between two devices (for example transistors, resistors, capacitors, etc.) which, though it is intended that they be identical, vary to one degree or another due to limitations in the fabrication processes.
Therefore, offset calibration with temperature resistance in a flash ADC is desired.